The activity of pulmonary vascular smooth muscle will be studied in either open-chest or chronically instrumented dogs. Previous experiments on the aorta have documented that vascular smooth muscle activity greatly affects the characteristic impedance, Zc, suggesting that the control of Zc is the physiological function of these smooth muscles. Similar observations have been made on the pulmonary artery. In these experiments, Zc will be derived in the time domain from the instantaneous pulmonary pressure-flow relationship during the ventricular ejection phase. In this manner, Zc will be obtained over a fairly large pressure range allowing one to arrive at definitive conclusions whether experimental changes in Zc are caused by smooth muscle activity or are passively due to a change in pressure. Simultaneously, with measurements of Zc, pulmonary pressure-diameter relationship curves will also be obtained using sonomicrometer techniques. Finally, a catheter-tip pressure-diameter gauge will be used to study the distensibility of smaller pulmonary arteries from 3 to 6 mm wide in order to assess smooth muscle activity in these vessels where ultrasonic techniques cannot be easily applied. It is the aim of this research to elucidate with these combined approaches the responses of pulmonary smooth muscle to physiological and pharmacological stimuli. This information will contribute to the understanding of pulmonary smooth muscle function within the integrated control of the cardiovascular system.